1. Field of the Invention
The present invention relates to a charge stop point detecting method, charge stop point detecting device, and a battery pack which detect the point for stopping charging a rechargeable battery based on capacity charged/discharged to/from the rechargeable battery.
2. Description of the Related Art
Conventional rechargeable batteries, typically lithium ion batteries, are charged typically by so-called constant-current/constant-voltage charging operation. In the constant-current/constant-voltage charging operation, the rechargeable batteries are first charged in a constant-current charging manner, and are then charged in a constant-voltage charging manner, in other words, the charging operation is switched from the constant-current charging manner to the constant-voltage charging manner, when the terminal voltage (hereinafter, occasionally referred to as battery voltage) reaches predetermined voltage which is previously set lower than the maximum rated voltage available to charge the rechargeable batteries. In the constant-current charging manner, the rechargeable batteries are charged at predetermined constant current. In the constant-voltage charging manner, the rechargeable batteries are charged at a constant voltage. If the battery voltage of a battery exceeds the maximum voltage, this may cause deterioration of the life and charge/discharge capacity of the battery. Also, the battery may catch fire. For these reasons, the battery is charged with the battery voltage being adjusted under the maximum voltage.
A full charge capacity (FCC) value of a rechargeable battery is updated every cycle of the charging/discharging operation based on the integrated value of the discharging current of the rechargeable battery which is discharged to discharge stop voltage (learning point voltage) after the rechargeable battery is fully charged. The remaining capacity of the rechargeable battery after being charged is calculated by subtracting the integrated value of charging/discharging current (charging current and discharging current) from FCC updated immediately before this calculation. In this integration, the discharging direction is defined positive. The charging/discharging current is detected by the voltage drop produced by a current detection resistor which is connected on the charging/discharging path of the rechargeable battery. However, it is difficult to detect the charging/discharging current within the range between about ±5 mA. For example, when a leak current flows from the rechargeable battery into a system such as external electric device, if this leak current cannot be detected as discharging current, FCC is updated to capacity smaller than the actual FCC. Accordingly, the remaining capacity is also calculated at capacity smaller than the actual capacity.
On the other hand, when a rechargeable battery is charged, if it is not detected that the rechargeable battery is fully charged for some reason, the rechargeable battery may be charged higher than the rated charge capacity. In this case, the rechargeable battery may be overheated, and may accidentally catch fire or explode.
In order to prevent this, technologies has been developed to monitor the charge capacity and to detect abnormalities in charging operation.
For example, Japanese Laid-Open Patent Publication No. JP 2002-325,363 A discloses a trouble checking method and a battery pack which calculates the capacity or the remaining capacity during charging operation of a rechargeable battery, and determine whether the calculated capacity exceeds substantially 1.5 to 2 times the rated capacity or learning capacity whereby checking trouble in the rechargeable battery and a charge control system.
However, in the technology disclosed in JP 2002-325,363 A, when the calculated FCC or capacity is smaller than the actual capacity as discussed above, it may be determined that trouble occurs in the rechargeable battery or the charge control system even if the rechargeable battery is properly charged.
The present invention is aimed at solving the problem. It is an object of the present invention to provide a charge stop point detecting method, a charge stop point detecting device, and a battery pack which can properly decide to stop charging a rechargeable battery even if calculated charged capacity or calculated remaining capacity is deferent from the actual capacity.